This invention is generally directed to processes for the preparation of particles, and more specifically, the present invention relates to processes for the preparation of polymers that can, for example, be selected as carrier coatings for xerographic developer compositions. In one embodiment, the process of the present invention relates to the preparation of polymers, such as polymethylmethacrylate by a modified aqueous emulsion polymerization wherein an emulsifier such as a block copolymer is selected as an emulsifier, and an alkali persulfate, such as potassium persulfate, is utilized as an initiator. The polymers obtained can be subjected to purification, and subsequently isolation by, for example, known methods such as freeze drying. The polymers obtained can be selected as carrier coatings, which coated carrier particles can be prepared by a dry powder process. The carrier particles obtained can be comprised of a core with coating thereover generated from a mixture of polymers that are not in close proximity thereto in the triboelectric series. These carrier particles can be prepared by a dry coating process wherein a mixture of certain polymers is applied to the carrier enabling insulating particles with ralatively constant conductivity parameters; and also wherein the triboelectric charge on the carrier can vary significantly depending on the coatings selected. Developer compositions comprised of the carrier particles prepared by the aforementioned dry coating process are useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging and printing processes. Additionally, developer compositions comprised of substantially insulating carrier particles are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core. The carrier particles with coatings thereover, wherein at least one of the coatings is comprised of the polymers obtained, with the processes of the present invention are illustrated in U.S. Pat. Nos. 4,937,166, and 4,935,326, the disclosures of which are totally incorporated herein by reference.
Polymer coatings are commercially available, for example polymethylmethacrylate is available from Soken Chemical and Engineering Company Limited. The aforementioned polymer can be prepared by, for example, known soap-free emulsion polymerization. In this process, there can be selected a small amount (about 1 percent) of acrylic acid which acts as an emulsifier. This water soluble monomer forms micelles where polymerization takes place, followed by grafting to the formed polymer, and functions, it is believed, as a steric stabilizer. There results a very well dispersed network of polymer particles which are very difficult to isolate. To accomplish this isolation, a small amount of zinc acetate is added to the dispersed network which serves to aggregate the particles, allowing for washing and drying. Isolation is then accomplished by jetting the aggregated particles, such that individual submicron polymer particles result, wherein the stabilizer, such as poly(acrylic acid), is not removed therefrom. Also, some zinc acetate remains in the product and yields can be very low (about 20 percent).
Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may be comprised of various cores, including steel, with a coating thereover of, for example, fluoropolymers, and terpolymers of styrene, methacrylate, silane compounds, a mixture of polymers not in close proximity in the triboelectric series, and the like. Recent efforts have focused on the attainment of coatings for carrier particles, for the purpose of improving development quality; and also to permit particles that can be recycled, and that do not adversely effect the imaging member in any substantial manner. A number of the present commercial coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there are disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of 20 minutes to about 120 minutes enabling the thermoplastic resin particles to melt and fuse on the carrier core.
Thus, for example, there can be formulated developers with conductivities of from about 10.sup.-6 mho (cm).sup.-1 to about 10.sup.-17 mho (cm).sup.-1 as determined in a magnetic brush conducting cell; and triboelectric charging values of from about a -8 to a -80 microcoulombs per gram on the carrier particles as determined by the known Faraday Cage technique, and wherein the carrier contains a first and second polymer coating, which polymers may be obtained with the processes of the present invention. Developers can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, maintaining the same coating weight on the carrier particles and changing the polymer coating ratios. Similarly, there can be formulated developer compositions wherein constant triboelectric charging values are achieved and the conductivities are altered by retaining the polymer ratio coating constant and modifying the coating weight for the carrier particles.
Other patents of interest include U.S. Pat. No. 3,939,086, which teaches steel carrier beads with polyethylene coatings, see column 6; U.S. Pat. No. 4,264,697, which discloses dry coating and fusing processes; and U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935 and 4,434,220.
In a patentability search report, the following prior art was recited: U.S. Pat. No. 4,286,037, which discloses the preparation of thermoplastic resin particles of a suitable size by coagulating an aqueous resin solution that contains one of the ions required for forming a quaternary ammonium salt in a coagulated liquid containing the other said ions, see the Abstract; also note column 5, especially line 35, wherein potassium persulfate, or a peroxide is disclosed as a polymerization initiator; and column 6, especially lines 47 to 51; and as some background interest U.S. Pat. Nos. 4,777,104, and 4,943,505.